The present invention relates to optoelectronic transmitters, and more particularly to optoelectronic transmitters having automatic power control and biasing circuitry for driving semiconductor lasers.
Optoelectronic transceiver modules provide an interface between an electrical system and an optical transfer medium such as an optical fiber. Correspondingly, most optoelectronic transceiver modules contain electrical and optical conversion circuitry for transferring data to and from the electrical system and the optical transfer medium.
Normally, transceiver modules use laser diodes which produce coherent light for performing high speed data transfers between the electrical system and the optical transfer medium. Typically, each laser diode is packaged with optical power-monitoring circuitry. For example, the HFE4081-321 diode package by Honeywell, Incorporated, contains both a laser diode for transmitting data and a photodiode for performing power-monitoring.
The power-monitoring photodiode within the diode packaging provides a monitor current Im which varies as the optical power being generated by the laser diode changes. Normally, the changes in the monitor current Im are directly proportional to the changes in the optical power generated by the laser diode. However, the ratio of monitor current Im with regard to the laser diode""s optical power can vary widely from one diode package to the next. Therefore, each diode package must be tested individually in order to determine its specific ratio of monitor current Im to laser diode optical power.
The primary purpose of providing a monitor current Im is for ensuring that, during operation, the laser diode is lasing. The minimum current which must be supplied to the laser diode to cause lasing is the threshold current Ith.
When the current being supplied to the laser diode is less than the threshold current Ith, the laser diode is operating in the LED mode. In the LED mode, the current supplied to the laser diode is only sufficient to excite atoms in the laser diode""s cavity which cause light to be emitted similar to that produced by light emitting diodes (LEDs).
When the current supplied to the laser diode reaches a level greater than or equal to the threshold current Ith, the laser diode""s efficiency of converting electrical current into light will increase dramatically and thus the laser diode changes from the LED mode of operation to the lasing mode of operation.
Referring to the drawings, FIG. 1 illustrates typical output power versus input current curves, or P-I curves, for three individual semiconductor lasers A, B and C. One of the primary difficulties with semiconductor lasers is that each individual laser has its own unique set of output characteristics. In FIG. 1, the horizontal axis (I) represents the drive current input to the semiconductor laser, and the vertical axis (P) represents the corresponding optical output power delivered by the laser. As can be seen, a uniform DC input current IQ supplied to each of the individual semiconductor lasers A, B and C results in a different amount of optical output power, PQA, PQB, and PQC, being delivered by each of the lasers. Furthermore, since the linear operating range for each semiconductor laser has a different slope, a given change in the input current xc2x1xcex94I will cause a different change in the output power xc2x1xcex94P for different semiconductor lasers.
These variations in the slope efficiency of the semiconductor lasers can be seen in FIG. 1. The uniform DC operating current, or quiescent current, IQ is applied to each of the three lasers A, B, and C, and an identical alternating current signal ISIG is superimposed thereon wherein Isig=IQxc2x1xcex94I. ISIG causes a periodic change in the input currentxc2x1xcex94I above and below the quiescent current IQ. The magnitude of the xcex94I applied to each semiconductor laser in FIG. 1 is identical between the three semiconductor lasers A, B, and C. On the output side, however, the resultant changes in the output power xc2x1xcex94PA, xc2x1xcex94PB and xc2x1xcex94PC generated due to the changes in the input current vary from one laser to the other. As is clear in FIG. 1, xcex94PA is greater than xcex94PB, and xcex94PB is greater than xcex94PC. These variations in the output characteristics of individual lasers raise a significant barrier to designing a standard, reliable optoelectronic transmitter for mass production.
Ideally, each optoelectronic transmitter of a particular design will have similar output characteristics. The optical output of the transmitter is to represent a binary data signal comprising a serial string of 1""s and 0""s. A binary 1 is transmitted when the optical output of the transmitter exceeds a certain power threshold, and a binary zero is transmitted when the optical output power of the transmitter falls below a certain power threshold. Maximizing the difference in transmitted power levels between 1""s and 0""s improves the reliability of the transceiver and improves the signal-to-noise ratio at the receiver input at an opposite end of a data link. Thus, in a transceiver design incorporating a semiconductor laser as the active optical element, the transmitter should include provisions for optimizing the output characteristics of the semiconductor laser. Furthermore, these output characteristics should be the same from one transceiver to another. The IEEE standard for Gigabit Ethernet is an example of a standard for data communications over an optical fiber which requires such uniform transmitter characteristics. Therefore, the optimizing circuitry should also normalize the output characteristics of the transmitter to a well-defined standard.
In generating the optical output signal, the transmitter driver circuit receives a binary voltage signal from the host device. The driver circuit converts the input voltage signal to a current signal that drives the semiconductor laser. A signal voltage corresponding to a binary 1 must be converted to a current supplied to the semiconductor laser sufficient to cause the semiconductor laser to radiate an optical output signal having an output power level above the power threshold corresponding to the transmission of a binary 1. Similarly, a signal voltage corresponding to a binary 0 must be converted to a current level supplied to the semiconductor laser which will cause the semiconductor laser to radiate an optical output signal having an output power level below the power threshold corresponding to the transmission of a binary 0. However, due to variations in the P-I characteristics from one semiconductor laser to another, the current levels necessary to produce the desired output power levels will vary depending on the individual characteristics of each individual semiconductor laser. U.S. Pat. No. 5,638,390, issued to Gilliland et al., discloses a design and method for stabilizing an optoelectronic transceiver having a laser diode. U.S. Pat. No. 5,638,390 is hereby incorporated by reference.
In general, variations in the P-I characteristics of individual semiconductor lasers can be compensated for by employing bias and AC drive circuits which adjust the input current driving the semiconductor laser. There are two components to the laser control circuits. The first component, automatic power control (APC), involves establishing the average DC input current, or quiescent operating current IQ. IQ establishes the average output power, or quiescent operating power PQ that will be radiated by the semiconductor laser. The second component, laser slope compensation, involves determining the change in the input current, +xcex94I, necessary to cause a desired change in the output power xc2x1xcex94P to establish the optical power levels corresponding to digital 1""s and 0""s respectively.
FIG. 2 shows the identical P-I curves for semiconductor lasers A, B, and C as shown as in FIG. 1. However, rather than the same input current being applied to each device, diverse input currents IA, IB, and IC are applied to each semiconductor laser A, B, and C respectively, such that each laser emits approximately the same output power PQ. APC and laser slope compensation are best described in conjunction with the P-I curves, as shown in FIG. 2. The optical power signal transmitted alternates above and below the quiescent output power PQ. Binary 1""s are represented as PQ+xcex94P, and binary 0""s are represented as PQxe2x88x92xcex94P. The ratio of power levels between 1""s and 0""s is the extinction ratio of the transmitter. A greater extinction ratio, meaning a higher ratio of the output power levels between transmitted 1""s and 0""s, results in improved receiver performance at the opposite end of the data link. Therefore, it is desirable to control PQ and xcex94P to maximize the extinction ratio RE. To maximize RE it is best to establish a quiescent operating power PQ near the midpoint of the operating range of the semiconductor laser. Once PQ has been established, the extinction ratio can be maximized by setting PQ+xcex94P as nearly as possible to the maximum output level of the semiconductor laser, and setting PQxe2x88x92xcex94P as nearly as possible the lasing power threshold of the semiconductor laser as possible.
As is clear from FIG. 2, the quiescent input current IQ, necessary to achieve the same, or nearly the same, quiescent output power PQ, will vary depending on whether semiconductor laser A, B or C is employed. Automatic power control establishes the quiescent input current IQ so that the desired average output power PQ is radiated by the particular semiconductor laser employed in the transceiver. Thus, for the three semiconductor lasers A, B and C depicted in FIG. 2, the quiescent operating currents IQA, IQB, and IQC will each deliver an output power of approximately PQ from semiconductor lasers A, B, and C respectively. Determining the proper quiescent current IQ for a particular semiconductor laser involves individually testing the semiconductor laser and varying the input current supplied thereto until the desired output power is achieved. Once the quiescent current IQ has been determined, an AC drive current can be generated which will be superimposed on the average DC input current to the laser equal to IQ.
Once the quiescent current has been established, it is necessary to compensate for the varying slopes of the P-I curves for the various semiconductor lasers. Because the slope of each P-I curve is different, the magnitude of change in the input current xcex94I necessary to cause a desired change in the output power xcex94P will vary from one semiconductor laser to another. Referring again to the P-I curves of FIG. 2, a laser current signal ISIG composed of the quiescent currents IQA, IQB, and IQC and the modulation currents xcex94IA, xcex94IB, and xcex94IC is supplied to each of the semiconductor lasers A, B, and C respectively. The peak magnitudes of the alternating current signals are represented by the quantities xcex94IA, xcex94IB, and xcex94IC. Clearly, xcex94IC is greater than xcex94IB, and xcex94IB is greater than xcex94IA, yet for each semiconductor laser the corresponding change in the output power xcex94P is approximately the same for each device. By controlling the peak magnitude of the input current signals xcex94IA, xcex94IB, and xcex94IC, the laser signal currents ISIG can be tailored to the specific slope characteristics of a particular semiconductor laser such that the peak change in the output power xcex94P can be set at or near the same levels even though the slope of the particular semiconductor laser may vary. This method produces a transmitter with consistent average power and extinction ratio.
Since the binary signals to be transmitted by the transceiver are optical representations of a voltage signal received from the host device, the driver circuit must convert the received voltage signal into a modulation current xcex94I and superimpose xcex94I onto the DC quiescent current IQ. In tailoring the AC current signal to a particular semiconductor laser, a laser slope compensation circuit establishes the peak magnitude of xcex94I resulting from changes in the input voltage signal. The slope compensated laser signal current ISIG will vary between IQ+xcex94I and IQxe2x88x92xcex94I, where IQ and xcex94I have been calculated to provide the desired extinction ratio for the end application. Thus, for example, if semiconductor laser B of FIG. 2 is employed, the slope compensation circuit must supply ISIGB having peak values of IQB+xcex94IB and IQBxe2x88x92xcex94IB, where the quantity xc2x1xcex94IB has been calculated to provide the correct change in output power xcex94P of semiconductor laser B.
APC and laser slope compensation are generally accomplished through the adjustment or selection of resistors included within the laser control circuit. When the required quiescent current IQ and the proper magnitude of the slope compensated current signal ISIG have been determined, the resistors can be sized so the laser control circuit supplies the proper current signal to the semiconductor laser. APC and laser slope compensation help to normalize the output characteristics of the optoelectronic transceiver so individual transceivers may be used interchangeably without having a noticeable effect on the overall data communication system.
A problem with implementing APC and laser slope compensation, however, is they complicate the manufacturing process and add cost to the final transceiver product. Individually testing the output characteristics of each semiconductor laser is time consuming. Individually calculating the size of each resistor to optimize the output characteristics of each device is time consuming. What is more, having individualized components prevents the main transceiver printed circuit boards from being manufactured in a completely automated fashion. Instead, individual resistors must be sized and soldered in place by hand or potentiometers adjusted by skilled operators, adding time and cost to the manufacturing process. A less expensive method is desired for providing APC and laser slope compensation in optoelectronic transceiver modules where semiconductor lasers are employed as the active optical element. Variations in rise and fall times of a laser transmitter may be compensated for by using a variable capacitor to balance the effects of parasitic capacitance and adjust for the different rise and fall times of the given semiconductor laser diode.
In addition to APC and laser slope compensation, in some applications it is also advantageous to monitor the output power emitted by the semiconductor laser to ensure that the laser is operating within safe limits. Because the optical energy emitted by a semiconductor laser has the potential to be harmful to the eyes if transmitted with sufficient power, it is prudent to provide a mechanism for disabling the laser when the output of the laser exceeds safe operating levels. Such a mechanism should prevent the drive current from reaching the laser, and should provide a signal to the host device indicating that a laser fault has occurred.
The present invention is for normalizing the average power and peak power deviation of an optical output signal of an optoelectronic transmitter when coupled into an optical transmission medium such as an optical fiber by adjusting the output voltage of a laser driver to normalize output power characteristics of the transmitter. The normalization compensates for variations in the output characteristics of optical devices, and variations in optical coupling of output signals to a transmission media. The invention also allows for the variations in rise and fall times of a laser transmitter to be compensated for by the adjustment of a trimmer capacitor.
According to a first aspect of the present invention, an optical transmitter drive circuit including automatic power control (APC) and laser modulation is provided by a variable output voltage differential receiver/driver integrated circuit (IC) coupled to a semiconductor laser, wherein the receiver/driver is responsive to data signals for generating an output voltage. The output voltage is controlled by a first potentiometer, which may be a digital potentiometer, on an integrated circuit in order to generate an AC current signal for summing with a DC current signal to provide a laser drive current signal. The laser drive signal is received by a laser transmitter having a laser diode for producing optical power over an optical transfer medium and a photodiode for producing a feedback signal in response to said optical power. The automatic power control (APC) circuit includes an error amplifier, a second digital potentiometer and a bias current drive transistor, wherein the error amplifier has inputs for both the feedback signal and a voltage reference to generate an output control signal. The second digital potentiometer affects the output control signal, and the bias current drive transistor is responsive to the output control signal for supplying bias current to the laser diode. According to another aspect of the invention, one or more of the digital potentiometers, error amplifier or voltage reference is included on said integrated circuit.
According to another aspect of the invention, a laser fault detection and latching feature are on the same integrated circuit as the above-mentioned APC circuit and laser slope compensation control potentiometer. The laser fault detection feature monitors the output power of the laser and generates a fault signal when the output power exceeds a predetermined level. When an excess power fault is detected, the fault latching circuit disables the laser and sends a fault signal to a host device. The laser fault latching feature further includes laser fault reset and safe power up circuitry. The reset circuitry allows the laser fault condition to be cleared in a safe manner, such that the optical output of the semiconductor laser does not exceed safe operating levels. The power up circuitry disables the laser while determining the status of the control circuit during the initial application of the DC power supply. Once the integrity of the circuit has been determined, the laser is enabled. This feature allows for hot pluggability of a transceiver module so that the module may be safely installed in a system while the system is already operating.
These and other objects, features and advantages will become more apparent in light of the drawings and accompanying text.